1. Field of the Invention
The present invention relates to an apparatus for controlling a hybrid electric vehicle carrying a rechargeable battery (hereinafter also referred to as a battery) as an auxiliary power source.
2. Description of the Related Art
Recently, so-called hybrid electric vehicles (HEV) are becoming widespread as low-pollution cars for meeting a social demand for reduction in environmental load, improvements in fuel consumption, a reduction in oil resource consumption, etc. Such a HEV comprises an electric motor for driving the vehicle, a rechargeable battery as a power source, an electric generator for charging the battery, and a prime mover for driving the electric generator, such as an engine. In HEVs, electric power from the battery is supplied to the electric motor which in turn drives the HEV, while the battery may be charged by electric power generated by the electric generator driven by the prime mover, or regenerative electric power from the electric motor during deceleration or braking. In HEVs, the battery needs to be maintained in a preferable state of charge so as to satisfy a request for power assist in driving. To this end, the energy of the battery needs to be effectively released (discharged) or absorbed (charged) by controlling the outputs of the electric generator and the electric motor depending on the running state of the vehicle.
A preferable state of charge for a battery in HEVs is such that a sufficient amount of electricity remains in order to meet a demand for electric power in starting or a request for power assist in running. On the other hand, it is important for a battery to retain sufficient space for charge so as to recover energy during deceleration or braking as efficiently as possible. For these reasons, the state of charge (SOC) or the remaining capacity of a battery is controlled by the charging and discharging of the battery so as to be within an intermediate region about a predetermined target value of SOC.
Conventionally, if a battery is continuously charged and discharged while keeping the SOC of the battery within the intermediate region of SOC, a memory effect, such as discharge memory and charge memory, occurs in the battery. The memory effect is responsible for inability to accurately control the SOC of the battery to fall within a predetermined intermediate region.
Specifically, when discharge memory is present during power supply in response to a demand in traveling a HEV, even if the SOC of a battery is within an intermediate region so that the battery can still discharge, the output of the battery is underestimated so that the SOC of the battery is detected to be below the intermediate region. In this case, a battery controller misjudges that the battery cannot respond to a request for power assist in traveling in a sufficient manner. Therefore, power assist is limited so that the battery cannot respond to the power assist request in a sufficient manner. Further, the battery is charged even though the actual SOC of the battery is within an intermediate region. Therefore, it is not currently possible to sufficiently meet social demands, such as an improvement in fuel consumption and a reduction in oil resource consumption.
When charge memory is present during regenerative charge during deceleration or braking of an HEV, even if the SOC of a battery is within an intermediate region so that the battery can be further discharged, the input of the battery is overestimated so that the SOC of the battery is detected to be above the intermediate region. In this case, the battery controller misjudges that it is difficult to recover energy which could be otherwise recovered during deceleration or braking of the HEV, such that the battery cannot respond to a request for energy recovery in traveling since the battery is discharged until the SOC reaches the predetermined intermediate region.
When one or both of discharge memory and charge memory are present, an SOC range in which a battery can be controlled is narrowed, leading to a reduction in so-called “drivability”.
Charge memory can be conventionally prevented by full charge or mild overcharge (forced charge) of the battery. In contrast, there has actually been no effective measure to prevent discharge memory. It is known that discharge memory can be eliminated by completely discharging one or more times. This technique has been applied to shavers. However, in a HEV system, when a battery is highly discharged, the battery cannot respond sufficiently to a request for power assist in traveling. Therefore, in HEVs, the SOC of a battery is often intended to be guided to a relatively higher level within a predetermined intermediate region. Thus, it has been difficult to overcome the discharge memory effect.